The present invention relates to a video tape recorder (VTR) system, and in particular to a low band converter for recording color signals converted in low band onto a video tape media of a VTR system.
In general, color signals for use in a VTR are converted in low band, so as to avoid the occurrence of beat phenomena resulting from interference between the color signals and other frequency modulation (FM) signals, and further to prevent jitter noises from affecting the color signals owing to the limitation of high-frequency characteristic by relative velocity and width of a video tape used in a particular type of VTR (i.e. 0.5-inch VTR). Therefore, a known VTR system for converting the color signals in low band is shown in FIG. 1.
Referring to the known system, an automatic color gain controller 80 (hereinafter, referred to as "ACC") receiving a color signal from narrow band filter (not shown), amplifies the color signal when its level is lower than a reference level, thereafter outputing into a burst amplifier 11 and a burst gate detector 12 (hereinafter, referred to as "BGD"). Then, the burst amplifier 11 amplifies with about 60 decibels (dB) only a burst signal of the output of ACC 80, thereafter outputing into a main balance modulator 18 (hereinafter, referred to as "MBM"). BGD 12 selects only the burst signal of the output of ACC 80 by burst window pulses (hereinafter, referred to as "BWP") provided from a burst gate pulse generator BGPG 10, thereafter outputing into killer phase comparators KPC 13, 14.
The ACC detector 81 detects direct-current (DC) level of BGD burst signal, to control a reference level of the ACC 80, and the KPC 14 compares the BGD burst signal with phase of an oscillation frequency signal of a first voltage-controlled-oscillator VCO 15, to control the oscillation frequency of the VCO 15 at a fixed frequency of 3.58 MHz. A circuit portion comprising a digital phase-locked-loop DPLL 21, a ring counter 22, a phase-rotation circuit HR 23, a 1/4-counter 24, a 1/2-counter 25, a 1/320-counter 26, a second voltage-controlled-oscillator VOC 27, and a record automatic frequency control phase comparator 28 (hereinafter, referred to as "AFC"), generates a 629 KHz pulse signal supplied from a synchronization separator (not shown), to output into a sub-balanced modulator 16 (hereinafter, referred to as "SBM") through the phase-rotation circuit 23.
Thereafter, the SBM 16 receives the 3.58 MHz pulse signal of the first VCO 15 and the 629 KHz pulse signal of the phase-rotation circuit 23, and by balance modulation thereof supplies a fixed frequency of pulse signal corresponding to a sum and a difference of the two inputs, to the MBM 18 through a high-pass filter 101. The MBM 18 receiving a 4.2 MHz pulse signal from the filter 101, also receives the amplified color signal from the burst amplifier 11 to carry out a balance modulation therefrom, and to convert in low band the received color signal into a color signal that coincides with the horizontal synchronization signal in phase and frequency, thereafter outputing into a color killer switching circuit 110 through a video recording amplifier 90. The KPC 13 receives the 3.58 MHz pulse signal of the first VCO 15 delayed in phase by 90 degrees, through a phase delaying circuit 17, and the output of BGD 12, and by comparing phases thereof, generates a color control signal into the color killer switching circuit 110. The color control signal switches on and/or off the color killer switching circuit 110 transmitting the color signal amplified in the amplifier 90, into the low pass filter 100 that outputs to a pre-amplifier (not shown) only color signals covering 629.+-.500 KHz of low-frequency bands from the received color signals thereof.
However, the above described known circuit for converting in low bands the input color signal so as to make coincidence a horizontal synchronization signal with a VTR system in both phase and frequency thereof, generally performs a two-stage modulation with the SBM 16 and the MBM 18. Hence, there often arises a case that the phase of color signals converted in low band does not coincide with that of the horizontal synchronization signal in the VTR system. In addition, this frequency results in phase mismatching in the video system.